1. Field of the Invention
The invention relates to a process for the preparation of organopolysiloxanes having silanol groups, in particular for the preparation of linear organopolysiloxanes functionalized with lateral silanol groups.
2. Background Art
As is generally known, in the simplest case silanol groups can be prepared by the aqueous hydrolysis of SiCl-containing organosilicon compounds. While in the case of the monomeric silanes it is relatively easy to obtain defined silanols and to isolate them in pure form in this way, the analogous reaction for the synthesis of polymeric organosilicon compounds, in particular those having lateral silanol functionalization, is substantially more difficult. It is true that it is possible in principle to obtain polymers functionalized laterally with silanol groups by cohydrolysis of mixtures consisting of dialkyldichlorosilanes and alkyltrichlorosilanes and optionally trialkylchlorosilanes. However, the products are not defined compounds having a uniform structure but rather a random mixture of more or less strongly branched hydrolysis products which also have a minor proportion of silanol side groups. The targeted synthesis of organopolysiloxanes having lateral silanol groups is therefore not possible in this way.
In principle, a further method for the synthesis of organopolysiloxanes containing silanol groups is the acid- or base-catalyzed hydrolysis of SiH-functional siloxanes. However, those skilled in the art are aware that defined compounds having a uniform structure can rarely be obtained in this manner, since the course of the reaction is not specific. Rather, the dehydrocondensation of already formed silanol groups with SiH groups still present, equilibrations and fragmentations of the polymer main chain of the starting material and the condensation of silanol groups, likewise catalyzed by acids or bases, take place as secondary reactions simultaneously with the desired formation of the silanol groups. As a result, a random mixture of more or less highly branched and partly bridged polymers is therefore obtained.
The classical hydrolysis processes are therefore not suitable for selectively synthesizing organopolysiloxanes functionalized laterally with silanol groups.